Ramayana
स तु वर्षसहस्राणि तपस्तप्त्वा महावने । यन्त्रितो नियमैरुग्रैश्चकार सुमहत्तपः ॥ 7-3-11 ॥
And he, performing penance for thousands of years in the great forest, restrained by severe vows, carried out exceedingly great austerities.
english translation